“Best in Show,” Christopher Guest’s documentary-style peek into the world of dog shows, is a worthy follow-up to the brilliant “Waiting for Guffman,” but it lacks the deeper points that made the earlier film work so well.

That said, it should also be noted that when “Best in Show” is funny, it is funnier than anything else in the universe. It’s just that, well, it’s not always funny.

The story is of several disparate characters who converge on Philadelphia for the 125th annual Mayflower Dog Show. We meet them in their home towns, and then follow them to Philly, where the second half of the film deals directly with the show itself (and, of course, the aftermath).

First we meet Meg and Hamilton Swan (Parker Posey and Michael Hitchcock), a J.Crew/LL Bean catalog-wearing couple who have a number of issues in their marriage, though they prefer to project them on their dog. Then there’s Gerry and Cookie Fleck (Eugene Levy and Catherine O’Hara), who must deal with the fact that before marrying Gerry, Cookie apparently slept with every man on earth.

Next is Harlan Pepper (Christopher Guest), a Southerner who owns a fishing shop and has a good ol’ hound dog. Also, he would like to be a ventriloquist. There’s a flamboyant gay couple, Scott (John Michael Higgins) and Stefan (Michael McKean), with a precious little shih tzu; their major goal is to defeat two-time champion Sheri Ann Cabot (Jennifer Coolidge), a gold-digging 30-something tart with an outrageously coiffed gigantic poodle.

As with “Guffman,” most of the dialogue is improvised, and Guest has a great cast for it. Levy came from Second City, and Hitchcock and Coolidge came from the Groundlings (the two best professional sketch-and-improv troupes in the country, in case you don’t follow such things). Guest himself and McKean were in the legendary “This Is Spinal Tap,” which of course had a similarly improvised faux-documentary style. (One missed opportunity here: Many fans would love to have seen Guest and McKean improvising together, but their characters don’t share the screen for even a moment.)

Levy and O’Hara are terrific as the Flecks, though perhaps the best scene involving them is when they stop to visit an old friend, played by Larry Miller, whose job is talk down potential suicide jumpers. (“They always jump,” he says before describing a specific incident in which the person’s head was knocked off by a gargoyle on the way down, and that when he hit the pavement, his guts burst out “like they were spring-loaded.”)

Posey and Hitchcock do not fare so well with their relationship, as their bickering soon stops being funny and just starts being bickering. The same goes for the gay couple: amusing at first, but so stereotyped as to be unreal.

Just when things are getting disappointing, in comes someone to save the day: Fred Willard, of all people, playing the dog show’s color commentator, Buck Laughlin. Buck knows absolutely nothing about dog shows (or even dogs, it would seem), and 99 percent of the things he says are perfectly hysterical. His foil is erudite Trevor Beckwith (Jim Piddock), who keeps a straight face through it all and tries to explain why the owners aren’t allowed to put a Sherlock Holmes hat and pipe on the bloodhounds, for example.

It’s almost as though Guest knew the film would falter at this point and called in Willard. And he rises to the challenge admirably, firing off one hilarious line after another. Before long, all he has to do is say, “Here’s a question, and this might be a little off the subject…” and we’re giggling in anticipation. (“How much do you think I could bench press?” is one of his non-sequitur queries for his co-host.)

Where the film falters is in not adhering to its own conceit: a documentary about a dog show. There are “interviews,” as well as footage of conversations, but it’s far more slick than a documentary would be, with more cameras, and cameras in places they wouldn’t (or couldn’t) be in a real documentary. It’s the sort of thing that will bug film purists, as it makes it seem more contrived, though I admit the average movie-goer may not notice (or care).

More important is the lack of depth among the characters. The film invites comparison to “Guffman” (the entire “Guffman” cast is here, plus several others), so that’s what we’ll do. “Guffman” was all about characters who were quirky, yes, but who also had an almost tragic dumbness about them: You pitied them while you were laughing. The “Best in Show” folks are funny, for the most part, but that’s as far as it goes.

Here’s why this is a problem: “Guffman” worked because of the humor of the characters, but also because the community-theater play they were putting on was in itself very funny (because it was so bad). Most people have had some experience with community theater, so they could relate; even if they didn’t, though, we all know someone who is passionate about something they suck at. So “Guffman” managed to be specific and universal at the same time, making it potentially funny to everyone.

The actual dog show in “Best in Show,” in contrast, is not especially funny, except for Willard’s commentary. Furthermore, the number of people who have had direct involvement or interest in a dog show is much smaller than with community theater. Yes, we all know someone who is obsessive about something strange — dogs, comic books, gardening, whatever — but since these characters are so superficial, that connection alone is not enough.

In other words, this film will be funniest to those who have actually participated in dog shows. Everyone else will find it extremely entertaining, often hilarious — but ultimately not nearly as resonating and realistic as its predecssor.